Luminescent lanthanide chelates have become a primary focus of investigation due to their highly unusual spectral properties (Gudgin-Dickson et al. (1995) Pharmac. Ther. 66:207-235; Selvin, P. R. (2002) Annu. Rev. Biophys. Biomol. Struct. 31:275-302; and Hemmila et al. (2005) J. Fluoresc. 15:529-542). These molecules have been used in wide variety of biochemical assays, including, for example, medical diagnostics, drug discovery, and as imaging tools in cell biological applications. Luminescent lanthanide chelates are especially useful as non-isotopic alternatives to conventional organic fluorophores in the applications where high background fluorescence is an issue. The unusual spectral (i.e., sharply spiked peaks) and temporal (i.e., long lasting emissions) properties of the luminescent lanthanide chelates can allow for (i) ultra-high sensitivity of detection (ii) facile, simultaneous monitoring of several analytes in the same sample mixture, and (iii) more information to be obtained from a given individual analyte in a sample.
A lanthanide probe can contain, for example, an organic fluorophore and a caged, or chelated lanthanide. The fluorophore moiety acts as an antenna, or sensitizer, which absorbs the energy of the excitation light and transfers it to the lanthanide in a radiation-less fashion. The antenna is required to “pump,” or activate the metal, since the absorbance of the lanthanide moiety is very low. The antenna-to-lanthanide energy transfer occurs only over a short distance (on the order of a few angstroms), which generally requires that the two moieties be tethered together.
Unfortunately, however, the process by which the fluorophore enhances lanthanide luminescence is not fully understood, which makes the optimization of the probe and its components difficult.
Despite a great demand for lanthanide probes for their use in a growing number of biologic assays, researchers often encounter a cost limitation. Such probes can cost up to $10,000 per mg, which is due, in large part, to laborious synthetic procedures. Thus not only is their a strong, unmet need for novel, more reactive, and more functionally delineated luminescent lanthanide probes, but also for cost-effective synthetic strategies that would allow for greater access to these reagents.